1. Field of the Invention
The invention relates to medical devices used in the magnetic resonance imaging (MRI) environment and in particular to a conductive cable that may be used to connect medical devices and other peripheral equipment to a patient.
2. Background of the Related Art
MRI has achieved prominence as a diagnostic imaging modality, and increasingly as an interventional imaging modality. The primary benefits of MRI over other imaging modalities, such as X-ray, include superior soft tissue imaging and avoiding patient exposure to ionizing radiation produced by X-rays. MRI's superior soft tissue imaging capabilities have offered great clinical benefit with respect to diagnostic imaging. Similarly, interventional procedures, which have traditionally used X-ray imaging for guidance, stand to benefit greatly from MRI's soft tissue imaging capabilities. In addition, the significant patient exposure to ionizing radiation associated with traditional X-ray guided interventional procedures is eliminated with MRI guidance.
MRI uses three fields to image patient anatomy: a large static magnetic field, a time-varying magnetic gradient field, and a radiofrequency (RF) electromagnetic field. The static magnetic field and time-varying magnetic gradient field work in concert to establish proton alignment with the static magnetic field and also spatially dependent proton spin frequencies (resonant frequencies) within the patient. The RF field, applied at the resonance frequencies, disturbs the initial alignment, such that when the protons relax back to their initial alignment, the RF emitted from the relaxation event may be detected and processed to create an image.
Each of the three fields associated with MRI presents safety risks to patients when a medical device is in close proximity to or in contact either externally or internally with patient tissue. One important safety risk is the heating that can result from an interaction between the RF field of the MRI scanner and the medical device (RF-induced heating), especially medical devices which have elongated conductive structures with tissue contacting electrodes, such as cables in pacemaker and implantable cardioverter defibrillator (ICD) leads, guidecables, and catheters. Thus, as more patients are fitted with implantable medical devices, and as use of MRI diagnostic imaging continues to be prevalent and grow, the need for safe devices in the MRI environment increases.
Exemplary interventional procedures include, for example, cardiac electrophysiology procedures including diagnostic procedures for diagnosing arrhythmias and ablation procedures such as atrial fibrillation ablation, ventricular tachycardia ablation, atrial flutter ablation, Wolfe Parkinson White Syndrome ablation, AV node ablation, SVT ablations and the like.
The foregoing procedures, among others, may require peripheral equipment such as electrophysiology recording systems, catheter tracking systems, external stimulators, surface electrocardiograms, 12-lead electrocardiograms, ablation generators, external defibrillators, pulse oximeters, various vital monitors and other devices and equipment in direct electrical contact with the patient. Conductive cables are used to connect these medical devices and peripheral equipment to the patient. In particular cables are used that operably connect medical devices and peripheral equipment to a patient's skin via a surface pad or patch. However, the RF-induced heating safety risk associated with cables in the MRI environment results from a coupling between the RF field and the cable. In this case several heating related conditions exist. One condition exists because the cable may electrically contact a patch adhesively or non-adhesively connected to tissue or skin. RF currents induced in the cable may be delivered through the cable into the tissue or skin, resulting in a high current density in the skin or tissue below the skin and associated Joule or Ohmic heating. Also, RF induced currents in the cable may result in increased local exposure to RF energy in nearby skin or other tissue, thus increasing the tissue's temperature. The foregoing phenomenon may be experienced as dielectric heating. Dielectric heating may occur even if the cable does not electrically contact tissue, for example if the cable was insulated from tissue. In addition, RF induced currents in the cable may cause Ohmic heating in the cable, itself, and the resultant heat may transfer to the patient. In such cases, it is important to attempt to both reduce the RF induced current present in the cable and to limit the current delivered into the surrounding skin and/or tissue.
Methods and devices for attempting to solve the foregoing problem are known. For example, high impedance cables limit the flow of current and reduce RF induced current; a resonant LC filter placed at the cable/patient interface may reduce the current delivered into the body through the cable, non-resonant components placed at the cable/patient interface may also reduce the current transmitted into the body; and co-radial cable sets may be used to provide a distributed reactance along the length of the cable thus increasing the impedance of the cable and reducing the amount of induced current.
Notwithstanding the foregoing attempts to reduce RF-induced heating, significant issues remain. For example, high impedance cables limit the functionality of the cable and do not allow for effective ablation, pacing or sensing. Resonant LC filters placed at the cable/patient interface inherently result in large current intensities within the resonant components resulting in heating of the filter itself, at times exceeding 200° C. Additionally, a resonant LC filter at the cable/patient interface can result in a strong reflection of the current induced on the cable and may result in a standing wave that increases the temperature rise of the cable itself and/or results in increased dielectric heating near the cable which in turn heats surrounding tissue to potentially unacceptable levels and may melt the catheter or lead body in which it is housed. Non-resonant components alone do not provide sufficient attenuation to reduce the induced current to safe levels. Additionally, the components will experience a temperature rise, if the conductor cross-sectional area is too small. While a cable with distributed reactance (i.e. coiled cables) can reduce the level of induced current on the cable, it does not sufficiently block the current that is induced on the cable from exiting the cable through points of electrical contact with skin or tissue. Thus, while coiled cables may work for certain short lengths or distances, in situations requiring longer lengths or distances, coiled cables do not by themselves provide enough impedance to block current.
Current technologies for reducing RF-induced heating in medical devices, especially those in which a conductive cable is used to connect a medical device and/or peripheral equipment to a patient, are inadequate. Therefore, new cable constructs are necessary to overcome the problems of insufficient attenuation of RF energy.